The use of teaching devices to teach and learn the concepts behind and mathematical operations involving fractions, mixed numbers and decimals are known. With some such devices the measurement of fractional lengths by the use of rulers of various kinds is employed.
The present invention relates to one such ruler wherein a triangular shaped prism with three faces has twelve scaled working edges. Four scaled working edges are located on each prism face with two such edges being adjacent each other along one of the prism's three edges. The length of the prism's edge is at least twice that of two of the adjacent scaled working edges, scaled numbers appear on different prism faces having a common prism edge and are inverted from each other. This permits a user to use a common prism edge against a paper or suitable drawing surface with the desired scale against the surface. A reference mark on the surface from one of the scales can easily be interrelated to other scales by turning the prism over to show those scales.